Neurostimulation is a category of medical devices that are used to transfer electric charge or electrical fields to tissue and result in a physiological change which benefits the patient, or performs a physiological measurement. Neurostimulation is used today in the cochlea, the retina, the peripheral nerve system, the spine, the brain and other parts of the body.
In a particular application of Neuromodulation, conductive electrodes are placed in contact with certain cortical brain structures in order to treat certain neurological conditions. In the case of stimulating the cortical surface, for example, as described in US. Pat. App. 2008/0045775, the stimulation may relieve the symptoms of Parkinson's Disease, other movement disorders, or psychiatric disorders. In the case of stimulating an associated region of the cortical surface, for example, as described in U.S. Pat. No. 7,774,068, the stimulation can treat the symptoms of movement disorders including restless leg syndrome. In the case of stimulating the temporal love of the cortex, for example, as described in US. Pat. App. 2007/0055320 or [Theodore, W. H., Fisher, R. S., “Brain stimulation for epilepsy”, Lancet Neurology, 3 (2), pp. 111-118, (2004).], the stimulation can treat the symptoms of temporal lobe epilepsy.
In the case where a cortical electrode array is used for recording and stimulation in long term therapy, an implantable pulse generator supplies the electrical signal to the electrode lead in contact with the brain structure. Additionally, the implantable pulse generator can record neural activity and electromagnetically transmit information outside the body. All components are placed surgically.
In the case where a cortical electrode array is used for recording and stimulation as a diagnostic tool, it may be placed temporarily on the cortex, for example for a few weeks, and then removed when no longer required. The information can be captured using wearable, or implantable, or semi-implantable, hardware.
In most prior art the electrode placed in contact with the cortex brain tissue has been metallic, disc like, and relatively large in size (e.g., 3 mm in diameter). In many cases, the electrodes are as large as the brain structures themselves. The large size of electrodes prevents specific and precise stimulation and recording of small brain targets which may be responsible for disease. The resulting large electric fields and associated current paths stimulate other structures of the cortex, and do not concentrate on the intended target. Furthermore, these large electrodes cannot be used to identify the targets of the brain by neural-recording because the area they cover is very large.
Additionally, in most prior art, cortical electrodes are placed on the surface of dura mater which is an electrically insulating biomaterial. Placing electrodes on the dura mater, so called epidural electrode placement, prevents efficient charge transfer to and from the brain region, rendering stimulation and recording less efficacious. For example, electric fields and associated current paths established by an epidural electrode will not concentrate electrical stimulation on the intended target. This prevents the effective delivery of potentially therapeutic or diagnostic neural stimulation. Additionally, for example, neural signals that epidural electrodes are trying to capture will be very weak on the dural surface, and therefore signal-to-noise ratio will be very low. This prevents the reliable recording of diagnostically or therapeutically useful neural activity.
Current techniques that determine placement of such relatively large electrodes are accomplished by first performing a craniotomy that can vary in size but is usually at least 10 mm in diameter and be as large as several centimeters. An electrode array is then placed upon the surface of the cortex. Some surgeons may create a flap of the dura mater and place the electrode array directly on the cortical surface. Recordings of neural activity can be made using the electrode array, from several electrode contacts. This process is complex, requiring a highly skilled surgeon to place the electrode array, and usually a highly skilled neurophysiologist to interpret the neural recording data. The large craniotomies that have to be performed put the patient at risk of infection and serious collateral injury.
Attempts have been made at developing microfabricated devices specifically designed to incorporate an array of microelectrodes which can stimulate small volumes of tissue on the cortex of the brain. Attempts have also been made to develop sub-dural penetrating microelectrodes for use on the cortex of the brain, for example, as described in U.S. Pat. No. 5,215,088, “Three-Dimensional Electrode Device” by Normann et al. Additionally, descriptions have been made in [Richard et al., “A neural interface for a cortical vision prosthesis”, Vision Research, 39, pp. 2577-2587, (1999)]. The prior devices however have not been able to easily translate to clinical use even though they have been available for more than a decade. This may be a result of the materials that are required to construct the device, because Silicon is a brittle material which may easily break during implantation or removal. Additionally, the reason for the lack of success may be because their functions do not provide enough additional information to the surgical team, because they only provide one electrode per penetrating shaft.
An important requirement for a successful outcome of cortical stimulation therapy, is the accurate placement of the stimulation and recording electrodes within the stimulation target area. Mislocation may result in unwanted side-effects, including sensory motor deficits. Additionally, a mislocated recording electrode will yield little or no relevant physiological data to the surgical team. Prior art procedures approximately localize the target by pre-surgical imaging and planning, for example through Trans-Cranial Magnetic Stimulation as described in [Komssi et al., “The effect of stimulus intensity on brain responses evoked by transcranial magnetic stimulation”, Human Brain Mapping, 21 (3), pp. 154-164, (2004)] to identify a region of therapeutic interest. The targets themselves may be only a few mm or less, and not be detectable through standard imaging techniques alone. Therefore exploratory surgical procedures involving acute stimulation, many times with the patient awake during the procedure, are necessary. Once the precise target area is located, the acute or chronic recording and stimulation electrodes can be implanted at the precise location.
Disadvantages of the current technology include extension of operation time by several hours, which can be an increased burden for the patient, who may be awake during such procedures, and extended cost associated with lengthier procedures which are a heavy financial burden on healthcare providers. Increased risk of surgical complications from bleeding or tissue damage caused by large craniotomies or repeatedly placed electrode arrays are a major risk of infection for the patient. Additionally, the possibility that chronic electrode arrays are not precisely located at identified target for any number of reasons, including further brain movement require that patients return to surgery.